Raymond C Givens

University of North Carolina at Chapel Hill, North Carolina, United States

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Publications (5)28.38 Total impact

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    ABSTRACT: Despite several studies suggesting that CYP3A5 expression can influence the extent of hepatic CYP3A-mediated inhibition, a systematic in vitro-in vivo evaluation of this potential clinically important issue has not been reported. Using representative probes from two distinct CYP3A substrate subgroups (midazolam, erythromycin), the inhibitory potency of fluconazole was evaluated in pooled human liver microsomes (HLM) with a low or high specific CYP3A5 content, in recombinant CYP3A enzymes (rCYP3A), and in healthy volunteers lacking or carrying the CYP3A5(*)1 allele. Fluconazole was a slightly more potent inhibitor of CYP3A activity in CYP3A5-HLM than in CYP3A5+ HLM with midazolam (K(i) of 15 and 25 microM, respectively) but not with erythromycin (IC(50) of 70 and 54 microM, respectively). In comparison, fluconazole was a much more potent inhibitor of rCYP3A4 than rCYP3A5 with both midazolam (K(i) of 7.7 and 54 microM, respectively) and erythromycin (IC(50) of 100 and 350 microM, respectively). As predicted from HLM, with i.v. midazolam, the average (+/- S.D.) in vivo K(i) (K(i,iv)) was significantly higher in CYP3A5(*)1 carriers (24 +/- 17 and 17 +/- 8 microM for homozygous and heterozygous groups, respectively) than in noncarriers (13 +/- 6 microM) (p = 0.02). With the erythromycin breath test, the average K(i,iv) was not different between homozygous CYP3A5(*)1 carriers (30 +/- 12 microM) and noncarriers (58 +/- 53 microM). In conclusion, the effect of CYP3A5 on hepatic CYP3A-mediated inhibitory drug-drug interactions is substrate-dependent, and HLM, rather than rCYP3A, are the preferred in vitro system for predicting these interactions in vivo.
    Full-text · Article · Feb 2008 · Drug metabolism and disposition: the biological fate of chemicals
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    Raymond Carlos Givens
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    ABSTRACT: Physiologic effects of cytochrome P450 3A activity
    Preview · Article · Jan 2007
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    ABSTRACT: The objective of this study was to determine whether the polymorphic expression of CYP3A5 could explain some of the inter-individual variability observed in the extent of CYP3A-based drug-drug interactions. Midazolam and erythromycin, representative of two distinct CYP3A substrate subgroups, were used as probe substrates. The inhibitory potency of fluconazole was evaluated in healthy African American volunteers genotyped as CYP3A5*1/*1, CYP3A5*1/*X or CYP3A5*X/*X (n=6 in each group; X = *3, *6 or *7 allele). Each volunteer received, by intravenous injection, midazolam (1 mg) and [14C N-methyl]erythromycin (3 Ci: 0.07 μmol), with and without oral fluconazole (400 mg). Hepatic CYP3A activity was evaluated as midazolam clearance or the erythromycin breath test. Using measured fluconazole plasma concentrations, the in vivo Ki towards the two substrates in the three different genotype groups was calculated. Individuals who carried at least one functional CYP3A5*1 allele had a significantly higher and more variable in vivo Ki for fluconazole towards midazolam (24 ±17 μM and 17± 8 μM for CYP3A5*1/*1 and CYP3A5*1/*X groups, respectively) than individuals genotyped as CYP3A5*X/*X (13 ±6 μM). No significant effect of CYP3A5 genotype on the in vivo Ki of fluconazole towards erythromycin was detected, although the Ki,iv in CYP3A5*1/*1 individuals was lower (30±12 μM) than that in CYP3A5*X/*X individuals (58 ± 53μM). Fluconazole was a more potent inhibitor of midazolam hydroxylation than erythromycin N-demethylation in vivo. These results demonstrate that the effect of CYP3A5 genotype on inhibitory drug-drug interactions is substrate dependant, supporting in vitro studies involving CYP3A5-expressing and nonexpressing human liver microsomes. This research was supported in part by the National Center for Research Resources (M01 RR00046) and the National Institute of General Medical Sciences (RO1 GM38149, RO1 GM63666, and PO1 GM32165).
    No preview · Conference Paper · Jan 2006
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    ABSTRACT: A single-nucleotide polymorphism (A6986G) in the cytochrome p-450 3A5 (CYP3A5) gene distinguishes an expressor (*1) and a reduced-expressor (*3) allele and largely predicts CYP3A5 content in liver and intestine. CYP3A5 is the prevailing CYP3A isoform in kidney. We report that, among renal microsomes from 21 organ donors, those from *1/*3 individuals had at least eightfold higher mean kidney microsomal CYP3A5 content and 18-fold higher mean CYP3A catalytic activity than did those from *3/*3 individuals (P = 0.0001 and P = 0.0137, respectively). We also report significant associations between the A6986G polymorphism and systolic blood pressure (P = 0.0007), mean arterial pressure (P = 0.0075), and creatinine clearance (P = 0.0035) among 25 healthy African-American adults. These associations remained significant when sex, age, and body mass index were taken into account. The mean systolic blood pressure of homozygous CYP3A5 expressors (*1/*1) exceeded that of homozygous nonexpressors (*3/*3) by 19.3 mmHg. We speculate whether a high CYP3A5 expressor allele frequency among African-Americans may contribute to a high prevalence of sodium-sensitive hypertension in this population.
    Full-text · Article · Oct 2003 · Journal of Applied Physiology
  • Raymond C Givens · Paul B Watkins
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    ABSTRACT: Many drugs exhibit variable efficacy and toxicity. Pharmacogenetics explores the genetic underpinnings of variable drug response. Pharmacogenetic testing is beginning to enter the clinic and will have a significant impact on the practice of clinical gastroenterology. Thiopurine S-methyltransferase screening, which will likely become routine for thiopurine recipients, illustrates the promise and limitations of pharmacogenetics. Testing for variation in other drug metabolism pathways may also become important. Pharmacogenetics will complement but not replace traditional methods for choosing drugs and for selecting dosing regimens for narrow-therapeutic-index drugs.
    No preview · Article · Aug 2003 · Gastroenterology